Fly ash generated in coal firing power plants etc. is used for materials for cement and artificial lightweight aggregates, concrete admixture, etc. However, when fly ash is used as concrete admixture, unburned carbon of fly ash will absorb AE water reducing agent etc., and will reduce the workability of the concrete. And, at the time of placing of the concrete, unburned carbon comes floating and there is harmful influence of a black part occurring in the connecting portion of the concrete. Further, when there was much unburned carbon of fly ash, there was also a problem that the quality of an artificial lightweight aggregate deteriorated. Therefore, only fly ash with little unburned carbon was used for the materials of cement etc., the fly ash with high unburned carbon content could not be used effectively and was processed for reclamation as industrial waste.
In order to solve such a problem, in the first patent document, a technology for removing unburned carbon of coal ash is disclosed. In this technology, after powdery coal ash is agitated and mixed with nonaqueous solvent of a low specific gravity rather than water and water, the mixture is settled and is separated into water layer including coal ash and nonaqueous solvent including unburned carbon.
Further, the second patent document discloses a method of improving recovery rate and efficiently removing unburned carbon of coal ash by adding ionicity collector as a collector after adding acid to water slurry of coal ash in the process of coal ash processing with a hydrophobic process, in which collector is added to the water slurry of coal ash to make the unburned carbon hydrophobic, and flotation separation process, in which frother is added to the water slurry to generate air bubbles and the unburned carbon is adhered to the bubbles for flotation.    Patent document 1: Japanese patent No. 3060665 gazette    Patent document 2: Japanese Patent Publication Heisei 8-252484 gazette